In the testing of underground tanks for small leaks of the order of 0.1 gallon per minute or less, it has been found that the change in volume of the liquid in the tank caused by a change in temperature during the period of the test can be greater than the change in volume due to the leak.
For example, the specifications of the National Fire Protection Association require that testing equipment must be capable of detecting a leak as small as 0.05 gallon per hour, or about 0.19 cubic inches per minute. However in a 5000 gallon tank full of gasoline which has a coefficient of expansion of about 0.00065 per degree F..degree., a change of temperature of only 0.01.degree. F. amounts to about 0.0325 gallons, or 7.5 cubic inches. If the change in temperature of 0.01.degree. F. occurs over a period of 1 hour, the change in volume will occur at the rate of about 0.125 cubic inches per minute. Since in the usual test method, the tank is filled shortly before the test is conducted, and since the temperature of the liquid added to the tank is almost always different from the temperature of the liquid already in the tank, which may itself be different from the surrouding ground temperature, rates of temperature change during the test usually exceed the above mentioned 0.01.degree. F. per hour. Also, since temperature gradients exist in the tank, measuring the temperature at one point in the tank, even at the mid-point, during the test does not give an accurate measure of the change in volume of the tank liquid due to temperature during the test. Since the temperature gradient is not uniform from the top to the bottom of the tank, even measuring the temperature at a various depths will not necessarily give an accurate measure of the change in liquid volume.